Introduce parseCommaSeparatedList

[makenowjust]

Discussed in #111 (comment)

This PR introduces a new function parseCommaSeparatedList. This function is a replacement of the following idiom to parse comma separated list:

	xxxs := []*ast.XXX{p.parseXXX()}
	for p.Token.Kind == "," {
		p.nextToken()
		xxxs = append(xxxs, p.parseXXX())
	}

In fact, this PR reduces about 60 lines of code.